Conditioning method for dehydrating clarification sludge

ABSTRACT

A process is described for dewatering sewage sludge consisting of the combination of an acidic oxidative preconditioning with an inorganic post-conditioning, in which case the preconditioning comprises an acidification of the sewage sludge and a catalytic partial oxidation by addition of a substoichiometric amount of hydrogen peroxide and iron ions at a pH≦5 and then an inorganic post-conditioning is carried out in which the acidified and partially oxidized sewage sludge is admixed with alkaline earth, and in which case, in the inorganic post-conditioning, sufficient calcium hydroxide (Ca(OH) 2 ) is supplied so that the pH of the limed sewage sludge is in the range from at least 9 to at most 11, in order thereafter to dewater the conditioned sewage sludge mechanically.

BACKGROUND OF THE INVENTION

Industrial and municipal wastewater treatment produce large amounts ofsewage sludges, generally as a mixture of “primary sludge” (PS) and“secondary sludge” (surplus activated sludge SAS). The sludges, toreduce/utilize the organic sludge fraction, are in part subjected to adigestion and even after their thickening are present as veryhigh-water-content raw sludges having low dry matter contents of, forexample, only 4 to 5% by weight. In Germany this involves annuallyapproximately 80 million tons of raw sludges, equivalent toapproximately 3.6 million tons of dry matter per year.

The invention is directed at a considerable improvement in dewatering ofindustrial and municipal sewage sludges and relates especially to anovel process for sewage sludge conditioning as an important processstep prior to actual sewage sludge dewatering.

Each sewage sludge conditioning treatment, such as the conventionalorganic conditioning with polyelectrolytes or else the conventionalinorganic conditioning with lime, is intended to ensure or improve thedewaterability of the sewage sludges (which are still rich in organicseven after digestion), since without a specific conditioning they wouldbe difficult to dewater. The purpose of the invention is a novel sewagesludge conditioning which leads to a considerable improvement in thesubsequent sewage sludge dewatering.

Every current sludge treatment has the purpose of utilization orelimination and generally proceeds via the processing substeps offlocculation/thickening, sludge reduction (for example digestion,hydrolysis) and/or—directly—flocculation/thickening, conditioning,dewatering, drying and incineration. Sewage sludge incineration plantsgenerally include the residual drying which is still required of themechanically dewatered sewage sludge (for example fluidized-bed furnacesand multiple hearth furnaces). The final purpose of sludge treatment is,according to the German Technical Regulations on Waste, generally sewagesludge mineralization by incineration.

Critical factors in the overall costs of sewage sludge mineralization byincineration are, in addition to the costs of the other upstream stages,the incineration costs. If it were to be possible to reduce considerablythe water load on the sewage sludge, these could be correspondinglyreduced both with respect to capital costs (for example smaller plantsize) and also with respect to operating costs (for example reducedconsumption of auxiliary fuel).

The purpose of much current work on conditioning with respect todewatering and on dewatering itself is a sludge which is dewatered ashighly as possible (see, for example, [1]). A main starting point forimprovements of mechanical dewatering is still the upstream conditioningof the sludges high in organics. The conditioning must be preciselymatched to the sludge present in the individual case itself and at thesame time to the dewatering technique employed (for example decanters,fire filter presses, chamber filter presses, membrane filter presses).

Conventional conditioning is preferably carried out using organicconditioning aids, for example polyelectrolytes (PE), but not rarelyalso using inorganic conditioning aids such as slaked lime and/or othersubstances promoting dewatering, for example pulverized coal or fly ashfrom coal combustion.

Especially in the case of inorganic conditioning, the amount of sludgedry matter originally present (SDMo) is noticeably increased by theamount of conditioning aids added (CA) to the higher amount of drymatter then present SDM+=SDMo+CA. The CA load of the original sludge drymatter yCA (as the ratio CA/SDMo) is set, for example, to values of 0.25kg of CA/kg of SDMo, frequently even substantially higher. This increasein the amount of original sludge dry matter by 25% or more is a reason(but not necessarily a valid one) for organic conditioning using PEfrequently having been preferred recently to inorganic conditioning.

In addition to the described organic or inorganic conditioning, evenearlier, other possible methods for sludge conditioning have beeninvestigated. One of the investigated possible methods relates tooxidative partial degradation of organic sludge components, inparticular the “slimy microbial sludge fractions”, which obviouslygreatly impede sludge dewatering. Such oxidative treatment of sewagesludges based on the Fenton reagent (hydrogen peroxide H₂O₂ and divalentiron ions Fe⁺⁺ as catalyst, added, for example in the form of FeCl₂) orbased on ozone are the subject of extensive work which also considersancillary questions such as AOX degradation or odour reduction (as aspecific partial degradation).

A substantial part of the prior art on H₂O₂ conditioning is described insummary in [2]. Despite many studies (cf., for example, [3], [4], [5]),although using H₂O₂ conditioning the dewatering results achieved onvarious dewatering machines were somewhat improved, the improvement wasnot substantial, that is to say the dewatering results, in our opinion,were still unsatisfactory.

The dewatering result is usually described by the dry matter contentachieved DM+(% by weight), i.e. the ratio DM+=water/(water+SDM+), whichalso includes the conditioning aid CA added in variable amounts in theinorganic conditioning (SDM+=SDMo+CA). What is termed the water loadingyH₂O of the original sludge dry matter, that is to say the ratioyH₂O=water/SDMo appears far more suitable for engineering evaluation ofdewatering.

Conditioning not only has a beneficial effect on the extent of thedegree of dewatering achieved, but also on the dewatering rate (e.g.filtration times, pressing times) and thus the output of a dewateringmachine used.

The effect of an oxidative conditioning on sewage sludge dewatering in,for example, belt filter presses and chamber filter presses which isbeneficial in trend, especially of acidic oxidative conditioning usingthe Fenton reagent, is therefore known in principle. However, contraryreports are found, for example relating to the type and amount of theiron catalyst or of corresponding transition metals. From themultiplicity of these studies on the acidic oxidative conditioning ofsewage sludges using the Fenton reagent, the procedure in principle andunit operations which participate are substantially known from earlierwork:

Acidification proceeds as far as starting pHs of from 3 to 4.

Preferably, H₂O₂/Fe⁺⁺ molar ratios≦5:1 are employed.

Most frequently, the cheaply available Fe(II)SO₄.7 H₂O (green vitriol)is used, which, however, in the event of a subsequent neutralizationwith lime, would have the disadvantage of considerable gypsum formation.

The H₂O₂ requirement is sludge-dependent; the minimum H₂O₂ requirementis 1.0 to 1.5% by weight of H₂O₂ (based on SDMo).

The oxidation reaction proceeds somewhat rapidly, as a function oftemperature (minimum time required 10 min).

Elevated temperatures (e.g. 60 to 90° C.) which are, however, associatedwith the disadvantage of considerable residual filtrate pollution forexample with COD, BOD, NH₄ ⁺, accelerate the oxidation.

In some work, specific post-conditioning was claimed explicitly.

However, the references previously made to post-conditioning areconcerned only with:

a) an additional organic PE post-conditioning, with reference frequentlybeing made to the high pressure sensitivity of H₂O₂-preconditioned andPE-post-conditioned sludges, and/or

b) a “neutralization” as increasing the pH with, for example, NaOH,Mg(OH)₂ or Ca(OH)₂ back to the vicinity of the neutralization point(avoidance of corrosion) and furthermore to a final pH of a maximum of8.5, the latter with the aim of rebinding heavy metals which havedissolved in the interim in the previously acidic environment and/orwith the aim of decreasing the COD/BOD residual pollution of the laterfiltrate.

The object underlying the invention is to utilize completely thebeneficial effects of acidic oxidative conditioning, which havepreviously only been partially utilized, in order to improveconsiderably the dewatering results.

The starting point in the present case was the fact that in an existingsewage sludge incineration plant for industrial sewage sludges, onlyapproximately 60% of the sewage sludge filtercake (previously dewateredon chamber filter presses (CFP) after lime conditioning) was able to beincinerated, whereas according to the German Technical Regulations onWaste, from April 1997 all sewage sludges were to be incinerated. Thesewage sludge filtercake dewatered previously on chamber filter pressescontained on average approximately 34% by weight of DM+ at a slaked limeloading yCA of about 0.25 to 0.50 kg of CA/kg of SDMo, equivalent to awater load yH₂O of 2.4 to 2.8 kg of H₂O/kg of SDMo. The purpose of thedevelopment work was therefore roughly to halve the water load in thesewage sludge filtercake and thus approximately double the throughput (tof SDMo/yr) of the existing sewage sludge incineration plant.

DESCRIPTION OF THE INVENTION

The novel process starts from an acidic oxidative preconditioning inwhich the sewage sludge is initially acidified and then, at a pH≦5 withaddition of divalent iron ions (Fe⁺⁺) and a substoichiometric amount ofhydrogen peroxide (Fenton reagent), a catalytic partial oxidation takesplace; this preconditioning (which is known in principle) is followedaccording to the novel process by defined post-conditioning withalkaline earths, after which, in the last step, mechanical dewateringtakes place. In detail, therefore, apart from modifications describedbelow, use is made of the following process steps which are known inprinciple:

a) Firstly, the high-organic-content sewage sludge is subjected to anacidic oxidative preconditioning in which the sewage sludge is acidifiedwith HCl and, at a pH<5, catalytic partial oxidation takes place withthe addition of divalent iron ions Fe⁺⁺ and a substoichiometric amountof hydrogen peroxide H₂O₂.

b) Then an inorganic post-conditioning takes place, in which theacidified and partially oxidized sewage sludge is admixed with alkalineearth, more precisely with elevation of the pH to at least 9.

c) In the last step the sewage sludge thus conditioned is mechanicallydewatered using known dewatering apparatuses.

The object is achieved according to the invention, with respect to theabove-described process steps, by means of the fact that after an H₂O₂preconditioning, organic post-conditioning with alkaline earth metaloxides takes place, preferably with calcium hydroxide (Ca(OH)₂), and inthis post-conditioning a pH in the range from at least 9 to at most 11is set.

The process is advantageously carried out in such a manner thathydrochloric acid is used for the acidification and the addition is setsuch that the pH of the acidified sewage sludge is between 3 and 4.

The catalyst used for the partial oxidation is preferably an FeCl₂solution which is already added to the acidified and degassed sewagesludge, in which case the amount of FeCl₂ added is adjusted within theorder of magnitude of 0.75 kg of FeCl₂ (100% pure)/kg of H₂O₂ (100%strength), equivalent to an H₂O₂/Fe⁺⁺ molar ratio of 5:1, to 1 kg ofFeCl₂ (100% pure)/kg of H₂O₂ (100% strength).

The process is preferably carried out in such a manner that during thepartial oxidation sufficient hydrogen peroxide is added so that in theoxidizing sewage sludge a redox potential of 200 mV to 500 mV,preferably from 350 mV to 450 mV, is maintained.

The partial oxidation at pHs of 3 to 4 thus induced proceeds rapidlyeven at relatively low temperatures in the range from 15° C. to 40° C.and is therefore preferably carried out between 20° C. and 30° C., thatis to say elevated temperatures are not necessary.

A preferred procedure of the process is, further, that during theinorganic post-conditioning sufficient calcium hydroxide is added sothat the pH of the post-conditioned sewage sludge is at a predeterminedpreset value in the range of at least 9 to at most 11.

According to a further development of the invention, the sewage sludgeis acidified in two series-connected vessels, the pH in the secondvessel being maintained at a predetermined preset value in the rangefrom 3 to 4, by adjusting the hydrochloric acid addition to the firstvessel.

In addition, the partial oxidation can also advantageously be carriedout in two series-connected vessels, in which case the redox potentialin the second vessel is kept at a predetermined preset value in therange from 200 mV to 500 mV by adjusting the hydrogen peroxide additionto the vessel.

Finally, the inorganic post-conditioning can also advantageously becarried out in two series-connected vessels, in which case the pH in thesecond vessel is kept at a predetermined preset value in the range from9 to 11 by adjusting the addition of calcium hydroxide to the firstvessel.

By means of the invention the following advantages are achieved:

a) Firstly, the described specific combination of an oxidativepreconditioning, in which a partial oxidation of the sewage sludge isperformed, with an intensive, i.e. more strongly inorganic, butminimized in lime, post-conditioning leads, with comparably shortfiltration times as in the case of a pure lime conditioning, to adrastic decrease in the water load to values of, for example, yH₂O=1.0to 1.2 kg of H₂O/kg of SDMo.

 This means—compared with the dewatering results in the case of purelyinorganic lime conditioning or organic PE conditioning—a considerableimprovement, even with the use of conventional dewatering machines, forexample chamber filter presses, high-dewatering-efficiency decanters.Optimum dewatering is achieved on high-dewatering-efficiency dewateringmachines, for example membrane filter presses.

b) The lime charging yCA may be considerably decreased (roughly halved),compared with the purely inorganic lime conditioning.

c) By means of the process the original dry matter content is decreased,for example by 20%, (SDM+<SDMo) or only slightly increased (SDM+approximately equal to SDMo), since during the acidification of thesludges, more carbonate CO₂ is frequently expelled than calciumhydroxide is subsequently added.

d) The increased costs due to hydrochloric acid and hydrogen peroxideare substantially compensated for in the procedure of the invention bydecreasing the lime consumption, compared with a pure lime conditioning.

The novel process consists of the following individual steps:

acidification of the prethickened raw sludge with HCl with addition ofiron chloride solution, sewage sludge degassing (CO₂),

partial oxidation of the acidified sewage sludge mixture containingadded catalyst with hydrogen peroxide as oxidizing agent,

lime post-conditioning by adequate pH elevation,

dewatering of the thus preconditioned and post-conditioned sewagesludge, preferably using chamber filter presses or membrane filterpresses.

The process was studied and tested using various non-digested industrialsewage sludges from two plants both in the laboratory and long-term (3weeks at 24 h/d) on a completely continuous pilot-plant unit (throughput70 kg of raw sludge/h) using pilot-plant filter presses (CFP and MFPhaving a filter area of 0.1 m²) and over 3 weeks daily on abatch-operated pilot-plant-scale operational unit (50 m³ vessels) usingan operational press (CFP having a filter area of 120 m²) or a mobilepilot-plant-scale press (CFP and MFP having a filter area of 1.76 m²).

Compared with the conditioning and dewatering processes used hitherto inthe two plants, considerably improved dewatering results, a decreaseddry matter load and, inter alia, a significant odour reduction werefound. Examples of the results achieved on chamber filter presses (CFP)and on membrane filter presses (MFP):

CFP Dewatering of the Raw Sludge LEV (Loss on Ignition Approximately52%)

a) with conventional lime conditioning: yH₂O=2.4 . . . 2.8 kg H₂O/kgSDMo

b) with conventional PE conditioning: yH₂O=2.2 kg H₂O/kg SDMo(large-scale works test)

c) with conditioning according to the process: yH₂O=1.5 . . . 1.6 kgH₂O/kg SDMo

MFP Dewatering of Raw Sludge LEV (Loss on Ignition Approximately 52%)

a) with conventional lime conditioning: yH₂O=1.7 kg H₂O/kg SDMo

b) with conditioning according to the process: yH₂O=1.1 . . . 1.3 kgH₂O/kg SDMo

CFP Dewatering of Raw Sludge DOR (Loss on Ignition Approximately 70%)

a) with lime conditioning (reduced chamber depth): yH₂O=2 kg H₂O/kg SDMo

MFP Dewatering of Raw Sludge DOR (Loss on Ignition Approximately 70%)

a) with conditioning according to the process: yH₂O=1.2 kg H₂O/kg SDMo

The high efficiency of the process is due in part to the dissolution ofthe carbonates present in the sludge, in part to the oxidated lysis ofbacterial cells and in part to the dissolution of slimy gel structureswhich otherwise make the filtration highly difficult, but not least,however, to the combination of the acidic oxidation preconditioningwhich is known in principle with the defined lime post-conditioning asan additional dewatering aid.

Finally, the multistage arrangement and control of the process is alsocritical, in the sense of carrying out each of the partial stepscompletely.

A maximum increase in the dewatering result is achieved when membranefilter presses are used.

The complete process, in its optimum form, consists of the followingsteps:

1. Prethickening

Prethickening, which belongs to the customary prior art, in thickeners,separators or strainers, to dry matter contents around or above 50 g/lis advisable in order to keep the suspension volumes treated small. Theuse of polymeric flocculents in this step should be restricted to aminimum, since the following steps are influenced as a result.

2. pH Reduction to pH=3 to 4 by Acidification

Hydrochloric acid (HCl) is used for the acidification, instead of theotherwise customary sulphuric acid, in order to avoid subsequent gypsumprecipitation. The acid consumption depends on the carbonate andhydroxide content of the raw sludge. For a typical industrial rawsludge, approximately 100 g of 100% strength HCl is required per kg ofSDMo. At a higher carbonate content, approximately 120 to 105 g of 100%strength HCl are needed per kg of SDMo. In an industrial plant, the HClshould be added to the piping upstream of the vessel(s) required for pHadjustment in the acidification, in order to facilitate subsequent CO₂degassing. The residence time required for homogenization and sufficientcarbonate dissolution is, in the case of a narrow residence timespectrum, about 20 min. For pH control, a vessel cascade circuit, whichhas a sufficient inertia, has proven itself here, as it has downstreamof the peroxide addition. For the HCl addition, no measurement of drymatter loading is necessary. Instead of this pH control is sufficient.

3. Outgassing

In order to ensure that during the later lime post-conditioning, calciumcarbonate is not reformed, the CO₂ released during the acidificationmust be removed from the suspension as completely as possible. For thispurpose, degassing ports are provided or separate degassing apparatuses(for example a degassing cyclone in piping system) are used, with whicha substantial degassing takes place.

4. Addition of the Catalyst in the Form of an FeCl₂ Solution

The required amount of Fe⁺⁺ is equivalent in order of magnitude to theaddition of 0.75 kg of FeCl₂ (100% pure)/kg of H₂O₂ (100% strength),corresponding to an H₂O₂/Fe⁺⁺ molar ratio of 5:1. The addition isperformed during the acidification. The dosage rate can be madeproportional to the controlled hydrogen peroxide dosage rate, so that noseparate measurement or control is necessary.

5. Hydrogen Peroxide Addition

Approximately 1.5% of hydrogen peroxide are required, based on theincoming SDMo loading. When the reaction is carried out in a two-vesselcascade having 20 min residence time/vessel, it has been found that astable redox value occurs at the outlet and that this can be used tocontrol the amount of peroxide added. Measurement of the SDMo loading inthe feed is not required here either. Even in the event of greatvariations in sludge feed concentration (g of SDMo/l) and sludge origin(SAS/PS ratio), owing to the control on the basis of redox potential,addition of peroxide in proportion to SDMo loading resultsautomatically.

If only one vessel is used, then residence times of the order ofmagnitude of 2 h per vessel are required (instead of 2×20 min), in orderto allow the reactions to proceed to completion.

6. Lime Addition

In the subsequent elevation of pH to from 9 to 11, yCA=0.1 to 0.12 kg ofCa(OH)₂/kg of SDMo are required, that is substantially less than in thecase of pure conventional lime conditioning (yCA≧0.25 kg of Ca(OH)₂/kgof SDMo). The lime is added under pH control. In this pH control,account must be taken of the fact that the pH is not established untilafter some residence time. In the pilot-plant unit, in the first stageof the two-stage liming, a control pH of 10 was used.

For the industrial solution, the sludge buffer required upstream of thedewatering replaces the second stirred tank.

7. Dewatering

For the mechanical dewatering, in the test of the process both chamberfilter presses and membrane filter presses were used. It has been foundthat the sludge conditioned according to the process described isconsiderably less pressure-sensitive than sludges conventionallypost-flocculated with polymer. This applies not only to the directeffect of shear stresses in pumps and piping (low sensitivity withrespect to flock size) but also to the pressure programme duringfiltration (steeper pressure ramp possible).

The use of membrane presses leads to higher dry matter contents or lowerwater loadings yH₂O in the filtercake and to shorter and chiefly moreaccurately reproducible filtration times.

It is essential here that the acidic oxidative preconditioning proceedsunder the following optimized conditions:

a) Clear separation of acidification and oxidative reaction with H₂O₂,in which case the divalent iron can be added (preferably as FeCl₂) asearly as the acidification. Such a separation must at the same timeinclude the provision of sufficient residence times in each case.

b) Maintenance of a pH of 3 to 4 during the oxidative partial reaction,that is to say appropriate decrease of the pH in advance byacidification with HCl to values<4, the pH decreasing still somewhatfurther by the addition of FeCl₂.

c) Maintenance of a sludge-dependant optimum H₂O₂ loading of, forexample, 0.015 kg of H₂O₂/kg of SMDo (at a relatively low SAS content inthe raw sludge) up to 0.080 kg of H₂O₂/kg of SDMo (in the case ofhigh-SAS raw sludges). This H₂O₂ loading is established by maintaining asludge-dependant optimum redox potential of 200 to 300 mV at relativelylow SAS contents in the raw sludge or up to 500 mV in the case ofSAS-rich raw sludges.

e) Choice of an optimum molar ratio H₂O₂/Fe⁺⁺ of e.g. 5:1.

Surprisingly, the good dewatering results on raw sludges were achievedat temperatures of 20 to 30° C., that is to say without a separatetemperature increase of the oxidative reaction. The temperature increaseof the oxidation stage to markedly higher temperatures, which wasconsidered necessary in the earlier studies (cf. [2]), is not necessary.

In addition, it has surprisingly been found that in the case of ahigh-organic sewage sludge having loss on ignition around 70% (high SAScontent) comparably good, sometimes even better, dewatering results wereachieved as in the case of a sewage sludge having losses on ignitionaround 50% (lower SAS content). According to experience, the dewateringresults with conventional conditioning are generally considerably poorerthe greater the loss on ignition and thus the organics content of thesludges. In contrast, in the case of a conditioning according to theprocess of the invention, even high-organic sludges can be dewateredoptimally. The reason is that with the oxidative conditioningessentially only the microbial SAS fraction is changed, in that its“slime coatings” are destroyed by oxidation; therefore, the higher theSAS content, the more effective the oxidative partial degradation canprove (comparatively).

The positive partial effect of the oxidative partial degradation(“destruction of slime coatings”) is finally, that is to say during thedewatering, only completely utilizable with respect to degree ofdewatering and dewatering rate if sufficient inorganic limepost-conditioning is also performed.

When the preconditioning was carried out stepwise in batch reactors orin a multistage self-controlling manner as a throughflow process througha multistage Konti plant, it was surprising that the oxidation reaction,which is rapid even at room temperature according to the literature,nevertheless had a marked time requirement and therefore not only themean residence time is of importance, but obviously also a sufficientlynarrow residence time distribution. This experience led to the (atleast) two-stage implementation of the actual oxidation with presettingof the sludge-specific redox potential in the second vessel as areference value for the controller preset value in the first vessel.

Similar experience argues for an (at least) two-stage arrangement of theupstream acidification using HCl with presetting of the sludge-specificfinal pH of the second vessel as a reference value for the controllerpreset value in the first vessel.

The process of the invention is suitable especially for dewateringhigh-organic sludges, in particular for dewatering sewage sludges havingaverage and relatively high microbial sludge contents or for sludgeshaving sludge contents of other animal origin (for examplegelatin-containing sludges).

The principal field of application is in the broad sector of sewagesludges from industrial and municipal wastewater treatment by means ofaerobic and/or anaerobic treatment stages.

REFERENCES

[1] 3rd GVC Congress on Oct. 14-16, 1996 in Würzburg:

“Verfahrenstechnik der Abwasser- und Schlammbehandlung” [Processengineering of wastewater and sludge treatment], Volume 1-3 publisher:GVC VDI-GeselIschaft Verfahrenstechnik und Chemieingenieurwesen,Postfach 101139, D-40002 Düsseldorf

[2] Patent DE 2838386 C2:

“Verfahren zur Entwässerung von organischem Schlamm” [Process fordewatering organic sludge]

Kurita Water Industries Ltd., Osaka/JP (Laid-open 3.1.1980)

[3] J. Pere, R. Alen, L. Viikari, L. Erikson

“Characterisation and Dewatering of Activated Sludge from the Pulp andPaper Industry”, Water Sci. Tech., Volume 28, No. 1 (1993), pp. 193-201

[4] A. Mustranta, L. Viikari

“Dewatering of Activated Sludge by an Oxidative Treatment”, Water Sci.Techn., Volume 28, No. 1 (1993), pp. 213-221

[5] J. Hermia, G. Gahier, E. Tamagniau, J. P. Wenseleers

“La filtrabilitè de boues urbaines condittionèes au peroxyded'hydroène”, Tri. Cebedeau, Volume 33, No. 444 (1980), pp. 469-477

EXAMPLES

The invention will be described in more detail below with reference tothe example of fully continuous plants for high sludge throughputs. Inthe figures:

FIG. 1 shows a diagrammatic flow chart of a plant having only one mixingvessel in each case in the acidification, oxidation and limepost-conditioning,

FIG. 2 shows a diagrammatic flow chart of a plant having in each casetwo series-connected mixing vessels in the acidification and oxidation,

FIG. 3 shows a schematic drawing of a possible plant modification usingstatic mixers for the acidification and the (starting) oxidation andotherwise having only one mixing vessel for the (remaining) oxidationand for the lime conditioning.

In FIGS. 1 and 2, degassing ports on the vessels for conducting away thecarbonate CO₂ released in the acidification have not been shownseparately. Correspondingly, in FIG. 3, the depiction of a degassingcyclone in the pipe mixer system outline has been omitted.

The mean residence time in the individual vessels in a processarrangement according to FIG. 1 is in each case approximately 2 h.According to FIG. 1, the raw sludge which has been thickened in advance(termed mixed sludge, consisting of PS and SAS, of, for example, 50 g ofSDMo/kg of raw sludge) is fed continuously at a rate of 100 t/h(equivalent to 5 t of SDMo/h) to the stirred vessel 1 in which theacidification with HCl is performed. For this purpose, technical-gradehydrochloric acid of 30% by weight HCl is added continuously from theHCl tank 2 to the influent mixed sludge. The dosage rate is set usingthe pH-dependant controller 3 so that in the vessel 1 a pH of 4,measured with the pH electrode 4, is maintained. The HCl stream addedunder automatic control for this is approximately 1500 kg of 30%strength HCl/h. In the reaction vessel 5, the Fe⁺⁺-catalysed partialoxidation of the acidified sewage sludge by H₂O₂ from the H₂O₂ reservoir10 takes place. The H₂O₂ stream added to the vessel 5 is establishedunder automatic control owing to the controller 8 a such that a redoxpotential of, for example, 350 mV (in the case of a mixed sludge ofaverage SAS content) is maintained in the reaction vessel 5, determinedthere by the measuring electrode 9. The associated FeCl₂ catalystsolution is already added with the raw sludge stream, that is to sayupstream of the stirred vessel 1, to the acidification stage, via theline 7 from the FeCl₂ reservoir 6. For this purpose, a proportionalcontroller 8 a/8 b is provided which adjusts the FeCl₂ stream inproportion to the added H₂O₂ stream. The H₂O₂ stream is approximately150 kg of H₂O₂ solution (50% strength)/h. The FeCl₂ stream isapproximately 280 kg of FeCl₂ solution (20% strength)/h. The oxidationin the reaction vessel 5 takes place at atmospheric pressure (1 bar) ata temperature of 20 to 30° C.

The partially oxidized sewage sludge leaves the reaction vessel 5 viathe line 12 and is then fed to the vessel 13 for the limepost-conditioning. The lime addition to line 12 takes place from theslaked lime reservoir 14 using a pH-dependant controller 15, as a resultof which a pH>9 (for example pH=10), measured with the pH electrode 16,is maintained in vessel 13. The slaked lime (calcium hydroxidesuspension) has a concentration, for example, of 20% by weight ofCa(OH)₂. The metering rate of the slaked lime is 3000 kg of Ca(OH)₂suspension/h, equivalent to 600 kg of Ca(OH)₂/h.

The SDMo stream discharged from the vessel 13 with the conditionedsewage sludge is, owing to the outgassing, oxidation and solutionprocesses, only approximately 4 t/h. However, the reduction in sludgedry matter (here 20%) is highly sludge-dependant.

The conditioned sludge at approximately 104 t/h is finally passed intofilter presses (not shown here) for dewatering. These can be, forexample, chamber filter presses or membrane filter presses. TheCFP-dewatered sewage sludge filtercake has a dry matter content ofapproximately 45% by weight. In the case of MFP dewatering, a dry mattercontent of approximately 48% by weight is achieved.

The conditioning plant according to FIG. 2—also designed for fullycontinuous operation—comprises, in contrast to the plant according toFIG. 1, two series-connected connected vessels in each case in theacidification and oxidation stages, that is to say the acidificationtakes place in the two reaction vessels 1 a and 1 b and the catalyticpartial oxidation in the two vessels 5 a and 5 b. The HCl is added in asimilar manner to FIG. 1. In this case the pH in vessel 1 b as targetvariable (pH electrode 4 b) governs the ph in vessel 1 a (pH electrode 4a) as intermediate variable of the metering pump (in the manner of acascade controller). The same applies correspondingly to the partialoxidation in vessels 5 a and 5 b, that is to say the H₂O₂ is also addedhere in a similar manner to FIG. 1. In this case the redox potential invessel 5 b as target variable (measuring electrode 9 b) governs theredox potential in vessel 5 a (measuring electrode 9 a) as intermediatevariable of the metering pump. The associated FeCl₂ catalyst solution isagain fed to the acidification stage (upstream of vessel 1 a); it isalso metered here in proportion to the H₂O₂ stream added under automaticcontrol (proportional controller 8 a/8 b).

The required mean residence time in vessels 4 a and 4 b and 5 a and 5 bis, for a process arrangement according to FIG. 2, only approximately 20min in each case.

The lime post-conditioning is performed in FIG. 2 as in FIG. 1 in onlyone large vessel 13 having about 2 h mean residence time. This stagealso, if there is no storage vessel before the downstream filterpresses, should better consist of two series-connected smaller vesselshaving appropriate control of the slaked lime addition.

FIG. 3 shows an accordingly modified conditioning plant which, however,is made up in principle in a similar manner to a plant according to FIG.1. The raw sludge is acidified here not in a vessel, but in a firststatic mixer 19, and correspondingly also the FeCl₂ admixture in astatic mixer 20. Hydrogen peroxide is then added and, in the furtherstatic mixer 21, mixed with the acidified catalyst-containing sewagesludge, where the partial oxidation begins. The post-reaction vessel 22serves so that the partial oxidation comes to an end in accordance withthe H₂O₂ supply provided. The partially oxidized sewage sludge is thenadmixed with slaked lime in vessel 23 (similar to FIG. 1) and then fedto a dewatering machine. The simplified process variant in the modifiedembodiment according to FIG. 3, if appropriate without controllers inthe individual stages, that is to say if appropriate operated with fixedmetering rates, is useful for raw sludges that vary very little inamount and composition (for example SAS/PS ratio).

Although the present invention has been described in detail withreference to certain preferred versions are possible. Therefore, thespirit and scope of the appended claims should not be limited to thedescription of the versions contained therein.

We claim:
 1. Process for dewatering sewage sludge including thecombination of an acidic oxidative preconditioning with an inorganicpost-conditioning, in which case the preconditioning comprises anacidification of the sewage sludge and a catalytic partial oxidation byaddition of a substoichiometric amount of hydrogen peroxide and ironions at pH≦5 and then an inorganic post-conditioning is carried out inwhich the acidified and partially oxidized sewage sludge is admixed withalkaline earth metal oxides, wherein in the inorganic post-conditioning,sufficient calcium hydroxide (Ca(OH)₂) is supplied so that the pH of thelimed sewage sludge is in the range from at least 9 to at most 11, inorder thereafter to dewater the conditioned sewage sludge mechanically.2. Process according to claim 1, wherein hydrochloric acid is used forthe acidification and addition of the hydrochloric acid is set such thatthe pH of the acidified sewage sludge is between 3 and
 4. 3. Processaccording to claim 1, wherein a catalyst used for the partial oxidationis an FeCl₂ solution, in which case the amount added is approximately0.75 kg of FeCl₂ (100% pure)/kg of H₂O₂ (100% strength) of the amount ofperoxide used or more.
 4. Process according to claim 1, wherein duringthe partial oxidation sufficient hydrogen peroxide is added so that inthe oxidizing sewage sludge mixture a redox potential of 200 mV to 500mV is maintained.
 5. Process according to claim 1, wherein during thepartial oxidation sufficient hydrogen peroxide is added so that in theoxidizing sewage sludge mixture a redox potential of 350 mV to 450 mV ismaintained.
 6. Process according to claim 1, wherein during the partialoxidation, a pH of 3 to 4 and a temperature of 15° C. to 40° C. ismaintained.
 7. Process according to claim 1, wherein during the partialoxidation, a pH of 3 to 4 and a temperature of 20° C. to 30° C. ismaintained.
 8. Process according to claim 1, wherein the addition ofcalcium hydroxide is controlled in such a manner that in thepost-conditioned sewage sludge a pH of a minimum of 9 to a maximum of 11is present.
 9. Process according to claim 1, wherein the acidificationis carried out in two sequential vessels (1 a; 1 b) and the pH in thesecond vessel (1 b) is predetermined as a preset in the range from 3 to4, in which case HCl is added to the first vessel (1 a) whose pH isadjusted as a direct control variable for addition of HCl in the mannerof a cascade controller such that the pH in the second vessel (1 b) iskept at a predetermined preset value.
 10. Process according to claim 1,wherein the oxidative reaction is also carried out in two sequentialvessels (5 a; 5 b) and the redox potential in the second vessel (5 b) iskept in the range from 200 mV to 500 mV, in which case the H₂O₂ is addedto the first vessel (5 a), the redox potential of which is adjusted as adirect control variable for the H₂O₂ addition in the manner of a cascadecontroller such that the redox potential in the second vessel (5 b) iskept at a predetermined preset value.
 11. Process according to claim 1,wherein the inorganic post-conditioning with alkaline earth metal oxidesis also carried out in two sequential vessels and the pH in the secondvessel is kept in the range from at least 9 to at most 11, in which casethe lime is added to the first vessel, the pH of which is adjusted as adirect control variable for the lime addition in the manner of a cascadecontroller such that the pH in the second vessel is kept at apredetermined preset value.
 12. Process according to claim 1, whereinthe inorganic post-conditioning with alkaline earth metal oxides iscarried out with slaked lime and is also carried out in two sequentialvessels and the pH in the second vessel is kept in the range from atleast 9 to at most 11, in which case the lime is added to the firstvessel, the pH of which is adjusted as a direct control variable for thelime addition in the manner of a cascade controller such that the pH inthe second vessel is kept at a predetermined preset value.